1. Field of the Invention
This invention pertains generally to endovascular delivery systems and methods, and more particularly to systems and methods for delivery of expandable stents.
2. Description of Related Art
Current endovascular procedures use a metallic expandable stent to expand a lesion or to cover a target lesion. There is an increase in the use of stents due to advancements in biomaterial technology. For example, intracranial aneurysms had been treated only by endovascular coiling or open-surgical clipping until recently; but now those aneurysms can also be treated by using a stent that reduces the blood flow into the target aneurysm and induces complete thrombosis. In order to successfully treat an aneurysm with a stent, it is crucial to cover the aneurysm lesion completely with the stent. Any uncovered area in the aneurysm neck may make the stent procedure ineffective. Therefore, if an uncovered area occurs, an additional stent may need to be placed to completely cover across the aneurysm lesion. This applies not only to aneurysms, but also to stenotic/narrowing lesions. An atherosclerotic plaque covered by a stent should bridge the proximal to the distal normal arterial segment. Thus, the capacity to accurately place a stent across the lesion is a critically important element in endovascular stenting procedure.
There are generally two different types of stents: a laser-cut stent and a braided stent. There are pros and cons for each type of the stent. The most significant limitation of a braided stent is a phenomenon called “foreshortening,” which is defined as the change in the length of the stent from a constrained (i.e. compressed) state to an unconstrained (i.e. expanded) state. Although the degree of foreshortening is most significant in a braided stent, it may occur in a micro-machined or laser-cut stent as well. The degree of foreshortening with certain kinds of braided stents can be as large as 100%. This foreshortening poses a challenge to the treating physicians to place a stent very accurately.
Referring to FIG. 1, a stent is generally delivered to a target lesion via a catheter and delivery wire 12. When a braided stent is squeezed in a delivery catheter, the stent becomes longer (elongated stent 10b). Once the stent is being pushed out of the delivery catheter, the stent expands in the target vessel and it becomes shorter (foreshortened stent 10a). This foreshortening phenomenon must be taken into account when a braided stent needs to be placed in a lesion with high accuracy. A physician must therefore anticipate the degree of foreshortening during the stent placement, which requires a certain amount of training and clinical experiences. Nevertheless, it is virtually impossible to achieve 100% accuracy as long as the procedure depends on “anticipation” or “experience.”
Another challenge for the accurate stent placement is tortuous anatomy. It is difficult to control the position of delivery catheter and also to anticipate the shape and position of a stent in a curvy target lesion. Endovascular stenting procedures are performed under a 2-dimensional fluoroscopic X-ray imaging guidance. Even with the use of multiple angle fluoroscopic images (for example, bi-plane digital subtraction angiography machine), there is an intrinsic challenge in making a real time estimate on the length of a lesion in a tortuous 3-dimensional anatomy projected onto a 2-dimensional fluoroscopic view.